Process for producing electrochemical device

ABSTRACT

A method for producing an electrochemical device which exhibits improved output performance and improved durability in thermal, chemical, and mechanical is provided. The electrochemical device includes a first electrode, a second electrode, and an ion exchange membrane held between the two electrodes. The method includes forming a catalyst layer containing a catalytic substance such as platinum and polyvinylidene fluoride and attaching ion exchange groups to the polyvinylidene fluoride in the catalyst layer. The resulting catalyst layer containing ion exchange groups is used for at least either of the first and second electrodes.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Japanese Patent Document No.P2003-018491 filed on Jan. 28, 2003, the disclosure of which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing anelectrochemical device.

The existing electrochemical device that is fueled by alcohol aqueoussolution is composed of an anode electrode, a cathode electrode, and anion exchange membrane formed from a polymeric solid electrolyte, all ofwhich constitute the MEA (Membrane & Electrode Assembly). The ionexchange membrane is usually made of monomer such as Nafion (aregistered trade mark of DuPont for perfluorosulfonic acid resin), whichhas a high ion conductivity. Nafion is used also as a binder in theelectrode. (See, for example, Japanese Patent Laid-open No. Sho 61-67787(from column 11, line 16, to column 15, line 7) and Japanese PatentLaid-open No. Sho 61-67788 (from column 12, line 9, to column 16, line2.)

The fuel for the electrochemical device is dominated chiefly bymethanol. Other fuels under study include hydrogen gas and organicmaterials such as ethanol, dimethyl ether (DME), and diethyl ether(DEE). (See, for example, Japanese Patent Publication No. Hei 3-208260,from column 3, line 1 to line 16, and FIG. 1.)

Promising among these electrochemical devices is the direct methanolfuel cell (DMFC), which uses methanol aqueous solution as a fuel fordirect reaction with the anode electrode. The DMFC is highly expected tobe a small portable fuel cell as a next-generation power source onaccount of its high energy density.

The electrochemical device such as direct methanol fuel cell mentionedabove is required to have improved output efficiency and durability.However, the direct methanol fuel cell is inferior to hydrogen fuelcells in thermal durability, chemical durability, and mechanicaldurability.

Nafion (registered trade mark) mentioned above or conventionalsulfonated fluorocarbon polymer is liable to dissolve in the alcoholfuel and hence it liberates catalyst particles due to poor bindingforce, resulting in a reduced output. The tendency toward dissolutioncauses not only the peeling of the catalyst layer but also the weakeningor breakage of the MEA film.

For this reason, the binder in the electrode and the ion exchangemembrane (or polymeric solid electrolyte membrane) should be formed froma material which is hardly soluble in alcohol fuel and which has a highproton conductivity.

SUMMARY OF THE INVENTION

The present invention is directed to a method for producing anelectrochemical device composed of a first electrode, a secondelectrode, and an ion exchange membrane held between these electrodes,which includes steps for forming a catalyst layer containing a catalyticsubstance and polyvinylidene fluoride and attaching ion exchange groupsto the polyvinylidene fluoride in the catalyst layer, with the resultingcatalyst layer containing ion exchange groups being used for at leasteither of the first and second electrodes.

The method according to the present invention starts with forming acatalyst layer from a catalytic substance and polyvinylidene fluoridewhich is insoluble in methanol and water. Subsequently, ion exchangegroups are attached to the polyvinylidene fluoride in the catalystlayer. And, the resulting catalyst layer which contains ion exchangegroups is used for at least either of the first and second electrodes.The catalytic substance firmly binds to the polyvinylidene fluoride andthe catalyst layer helps the electrochemical device to retain goodoutput performance for a long period of time. In addition, theabove-mentioned procedure (in which ion exchange groups are attachedafter the catalyst layer has been formed) makes it easy to form thecatalyst layer. By contrast, conventional sulfonated fluorocarbonpolymer such as Nafion (registered trade mark) is soluble in alcoholfuel and electrochemical devices incorporated with it cause thecatalytic substance to liberate and hence deteriorate in outputperformance. The present invention has improved output efficiency inaddition to improved durability, such as thermal durability, chemicaldurability, and mechanical durability.

Moreover, being insoluble in methanol aqueous solution, thepolyvinylidene fluoride mentioned above may be incorporated into anelectrochemical device (such as direct methanol fuel cell) without thepossibility that the catalyst layer peels off and the MEA (Membrane &Electrode Assembly) film breaks during operation. MEA is composed of thefirst and second electrodes and the ion exchange membrane.

Thus, the present invention contributes to an electrochemical devicehaving improved output performance and improved durability in thermal,chemical, and mechanical.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic sectional view showing the method for producing anelectrochemical device according to one embodiment of the presentinvention.

FIG. 2 is a schematic sectional view of the electrochemical device.

FIG. 3 is a graph showing IR spectra obtained from the MEA before andafter treatment in the example of the present invention.

FIG. 4 is a graph showing the relation between the current density andthe elapsed time.

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, the method for producing anelectrochemical device includes forming a catalyst layer containing acatalytic substance and polyvinylidene fluoride (PVDF) and attaching ionexchange groups to the PVDF in the catalyst layer. The resultingcatalyst layer containing ion exchange groups may be used for at leasteither of the first and second electrodes. The procedure shouldpreferably be carried out by bonding the catalyst layer to a precursorof ion exchange membrane composed of polyvinylidene fluoride andbringing the bonded body into contact with a compound containing ionexchange groups, thereby introducing (through substitution) ion exchangegroups into the polyvinylidene fluoride in the bonded body.

The foregoing procedure will be described below more specifically withreference to FIG. 1, which is a schematic sectional view showing themethod for producing an electrochemical device according to oneembodiment of the present invention. The first step is to bond togetherthe catalyst layer 1 a which contains a catalytic substance such asplatinum and polyvinylidene fluoride and the precursor of ion exchangemembrane 2 a which is composed of polyvinylidene fluoride, therebyforming the bonded body 3 which functions as the MEA. In other words,the bonded body 3 includes the current collector 4 which functions asthe first electrode, the catalyst layer 1 a, the precursor of ionexchange membrane 2 a, the catalyst layer 1 a, and the current collector4 which functions as the second electrode.

Then, the bonded body 3 is dipped in a solution of a compound containingion exchange groups with heating under pressure, so that the compoundinfiltrates into the catalyst layer 1 a and the precursor of ionexchange membrane 2 a. In this way, fluorine atoms in the polyvinylidenefluoride are substituted by the ion exchange groups. Dipping conditionsmay vary as follows depending on the thickness and composition of thebonded body 3.

-   -   Concentration of solution: 5 to 10 mol/L    -   Pressure: 202650 to 303975 Pa    -   Temperature: 120 to 140° C.    -   Duration of dipping: 60 to 600 min

In this way, there is obtained the MEA film 5 which is composed of thecatalyst layers 1 b (each composed of the polyvinylidene fluoridecontaining ion exchange groups and the catalytic substance), the ionexchange membrane 2 b (composed of the polyvinylidene fluoride havingion exchange groups), and the current collectors 4.

Incidentally, the polyvinylidene fluoride should preferably have aweight-average molecular weight of about 1.0×10⁵ to about 1.0×10⁶.

Examples of the ion exchange groups include sulfonate group (—SO₃H),carboxyl group (—COOH), phosphate group (—PO₃H), linear sulfone group(—(CH₂)_(n)SO₃H, n=integer), perfluorocarbon liner sulfone group(—(CF₂)_(n)SO₃H, n in n integer) and the like. The ion exchange capacity(IEC) should preferably be about 0.9 to about 2.0 meq/g, more preferablyabout 0.9 to about 1.2 meq/g. This concentration is achieved easily andquantitatively by controlling the concentration of the solution.

Examples of the catalytic substance include platinum, ruthenium,palladium, silicon, carbon, aluminum, magnesium, cobalt, iron, nickel,molybdenum, and tungsten, which are known well. The catalytic substanceshould be used in an amount of about 0.15 to about 1.0 gram per g ofcatalyst or about 0.1 to about 2.0 mg per cm² of its support. Thecatalyst should have a particle diameter of about 1 to about 20 nm. Thecurrent collector 4 (as the first and second electrodes) may be made ofany well-known material such as carbon.

According to the present invention in an embodiment, the method forproducing an electrochemical device includes forming the catalyst layer1 a from polyvinylidene fluoride (insoluble in methanol and water) and acatalytic substance, bonding the catalyst layer to the precursor of ionexchange membrane 2 a, and dipping with heating under pressure thebonded body 3 in a solution of a compound containing ion exchangegroups, thereby introducing (through substitution) the ion exchangegroups into the polyvinylidene fluoride as the constituent of thecatalysts layer 1 a and the precursor of ion exchange membrane 2 a. Theelectrochemical device produced in this manner retains good outputperformance owing to the catalytic substance firmly adhering to thecatalyst layer.

Since the polyvinylidene fluoride is insoluble in methanol aqueoussolution as mentioned above, the MEA film 5 can be used as anelectrochemical device (such as direct methanol fuel cell) without thepossibility that the catalyst layer 1 b peels off or the MEA film 5breaks.

The thus obtained electrochemical device has improved output efficiencyand improved durability in thermal, chemical, and mechanical.

The electrochemical device to be produced by the method of the presentinvention should preferably be a fuel cell.

FIG. 2 is a schematic sectional view of a fuel cell as theelectrochemical device produced by the method of the present invention.

This fuel cell is made up of a cathode 7 (fuel electrode or hydrogenelectrode) having a terminal 6, an anode 9 (oxygen electrode) having aterminal 8, and an ion exchange membrane 2 as an electrolyte heldbetween the two electrodes. The cathode 7 and anode 8 each has acatalyst layer 1.

The multi-layered film (MEA) composed of the cathode 7, anode 9, and ionexchange membrane 2 may be formed by the method for producing anelectrochemical device according to the present invention.

First, the catalyst layer containing a catalytic substance such asplatinum and polyvinylidene fluoride is formed on current collectorssuch as carbon sheet, each functioning as the cathode 7 and the anode 9.Then, the precursor of ion exchange membrane composed of polyvinylidenefluoride is held between and bonded to the cathode 7 and the anode 9 (towhich the catalyst layer has been attached) such that it comes intocontact with the catalyst layers.

Subsequently, the resulting bonded body is dipped with heating underpressure in a solution of a compound containing ion exchange groups, sothat the ion exchange groups are introduced through substitution, intothe polyvinylidene fluoride constituting the catalyst layer and theprecursor of ion exchange membrane. In this way there is obtained themulti-layered film (MEA film) composed of the cathode 7, the anode 9,and the ion exchange membrane 2.

The fuel cell mentioned above works in the following manner. The cathode7 is supplied with methanol aqueous solution through the passage 10 formethanol aqueous solution passage. Hydrogen ions are liberated from themethanol (or fuel) while the methanol aqueous solution is passingthrough the passage 10, and migrate to the anode 9 with the hydrogenions liberated at the ion exchange membrane 2, at which they react withoxygen (air) passing through the oxygen passage 11. This chemicalreaction generates the desired electromotive force.

The foregoing structure may be modified such that an integral unit isformed from more than one MEA film composed of the cathode 7 (with thecatalyst layer 1), the ion exchange membrane 2, and the anode 9 (withthe catalyst layer 1). Such a modified structure produces a higherelectromotive force easily. The methanol aqueous solution as a fuel(which is supplied through the passage 10 as explained above) can bereplaced by hydrogen gas.

The fuel cell mentioned above is produced by preparing a catalyst layerfrom polyvinylidene fluoride (insoluble in methanol and water) and acatalytic substance, bonding the catalyst layer to a precursor of ionexchange membrane composed of polyvinylidene fluoride, and dipping withheating under pressure the bonded body in a solution of a compoundcontaining ion exchange groups, thereby introducing (throughsubstitution) the ion exchange groups into the polyvinylidene fluorideconstituting the catalyst layer and the precursor of ion exchangemembrane. The fuel cell produced in this manner keeps the catalyticsubstance firmly adhering and retains good output performance for a longperiod of time.

Being insoluble in methanol aqueous solution, the polyvinylidenefluoride can be applied to the direct methanol fuel cell without thepossibility that the catalyst layer 1 peels off and the MEA film breaksduring operation.

Consequently, the resulting fuel cell has improved output performanceand improved durability.

The foregoing description is based on the assumption that theelectrochemical device will be used as a fuel cell. However, theelectrochemical device can also be used as a hydrogen producingapparatus which is based on the principle opposite to that of fuelcells. It may also be used as a lithium cell which employs a solidelectrolyte capable of conducting lithium ions, a water electrolyzerwhich employs a solid electrolyte capable of conducting protons, or aproton pump.

The foregoing deals with the process for producing the MEA film 5composed of the catalyst layers 1 b containing ion exchange groups, theion exchange membrane 2 b, and the current collectors 4, by forming thebonded body 3 and then dipping the bonded body 3 in a solution of acompound containing ion exchange groups, thereby introducing (throughsubstitution) the ion exchange groups into the polyvinylidene fluoridein the bonded body 3, as shown in FIG. 1. The method of the presentinvention may be applied to an electrochemical device in which at leasteither of the first and second electrodes has the catalyst layer 1 bcontaining ion exchange groups.

Moreover, the ion exchange membrane 2 b held between the electrodes maybe any one of Nafion (registered trade mark) (perfluorosulfonic acid),non-fluorocarbon sulfonic acid, partially fluorinated carbon sulfonicacid, perfluorocarboxylic acid, non-fluorocarbon carboxylic acid,partially fluorinated carbon carboxylic acid, perfluorophosphoric acid,non-fluorocarbon phosphoric acid, partially fluorinated carbonphosphoric acid and the like. In this case, the other catalyst layer maybe formed from other polymeric compounds such as Nafion (registeredtrade mark). Alternatively, it is also possible to bond the catalystlayer 1 b, which has undergone dipping, to a separately prepared ionexchange membrane.

The invention will be described in more detail with reference to thefollowing Examples, which demonstrate the method for producing theelectrochemical device according to various embodiments of the presentinvention.

EXAMPLE 1

A solution was made by dissolving polyvinylidene fluoride (PVDF, fromAldrich) having a molecular weight of 150000 in NMP(1-methyl-2-pyrrolidone).

To this solution was added a catalyst for fuel cell (Pt—Ru/C, fromTanaka Precious Metals) such that the ratio of PVDF to the catalyst is0.6:1.0. After stirring for 24 hours, there was obtained a dispersionfor the anode catalyst. The catalyst for fuel cell is composed of Pt,Ru, and C in a ratio of 23:22:55 by weight. A dispersion for the cathodecatalyst was prepared in the same way as above from a catalyst for fuelcell composed of Pt and C in a ratio of 0.46:0.54 by weight (from TanakaPrecious Metals).

The solution of PVDF was cast alone onto a polyimide film (20 μm thick).After drying at about 40° C. under normal humidity, there was obtained aPVDF film (about 50 μm thick).

The dispersions for the anode catalyst and cathode catalyst wereindividually applied to a carbon sheet (from Electrochemical). Upondrying, there were obtained the cathode electrode and the anodeelectrode, each carrying 1.0 mg of Pt per cm².

The cathode and anode electrodes thus obtained were placed on both sidesof the PVDF film, and they were bonded together by hot pressing at 100°C. and about 30 kgf/cm² for 5-10 minutes. Thus there was obtained anuntreated MEA.

The thus obtained untreated MEA was dipped in an aqueous solution ofmethane sulfonic acid (1 M) and heated at 130° C. and 202650 Pa (2 atm)in an autoclave. Thus there was obtained a treated MEA in which PVDF wasmethane sulfonated. The treated MEA was washed with pure water to removeexcess methane sulfonic acid. The MEA was tested for fuel cellperformance in the following manner.

Gas for the oxygen electrode: atmospheric air, 100% humidity, 40° C.,100 mL/min

-   -   Fuel for the fuel electrode: 1 M MeOH aqueous solution    -   Electricity generation without reflux: 0.3 V, continuous    -   Current density was measured at t=0 and at intervals of five        minutes.    -   Ambient temperature: 22° C.    -   Relative humidity: 51%

It is clear from FIG. 3 showing the IR spectra of the untreated MEA andtreated MEA that sulfonic groups (—SO₃H) have been introduced (throughsubstitution) into PVDF after treatment.

COMPARATIVE EXAMPLE 1

The same procedure as in EXAMPLE 1 was repeated to produce the MEAexcept that the electrolytic film (ion exchange membrane) was replacedby “Nafion 112 (registered trade mark)” of the same thickness and thePVDF for the anode electrode and cathode electrode was replaced by“Nafion” (Furuuchi Chemical, EW1100, SE20192). The resulting MEA wastested for fuel cell performance.

FIG. 4 shows the output (changing with time) measured in EXAMPLE 1 andCOMPARATIVE EXAMPLE 1.

It is apparent from FIG. 4 that since, according to an embodiment of thepresent invention, the electrochemical device (fuel cell) is produced bypreparing the catalyst layer from polyvinylidene fluoride (insoluble inmethanol and water) and the catalytic substance, bonding the catalystlayer to a precursor of ion exchange membrane composed of polyvinylidenefluoride, and dipping with heating under pressure the bonded body in asolution of a compound containing ion exchange groups, therebyintroducing (through substitution) the ion exchange groups into thepolyvinylidene fluoride constituting the catalyst layer and theprecursor of ion exchange membrane, the resulting fuel cell keeps thecatalytic substance firmly adhering and retains good output performancefor a long period of time.

Being insoluble in methanol aqueous solution, the polyvinylidenefluoride can be applied to the direct methanol fuel cell without thepossibility that the catalyst layer peels off and the MEA film breaksduring operation.

This contributes to fuel cells having improved output performance andimproved durability.

According to an embodiment of the present invention, the catalyst layercontaining a catalytic substance and polyvinylidene fluoride (insolublein methanol and water) is formed and then ion exchange groups areattached to the polyvinylidene fluoride in the catalyst layer and theresulting catalyst layer containing ion exchange groups is used for atleast either of the first and second electrodes. Therefore, thepolyvinylidene fluoride firmly adheres to the catalytic substance andthe resulting electrochemical device retains outstanding outputperformance for a long period of time. Moreover, since the ion exchangegroups are attached after the catalyst layer has been formed, thecatalyst layer can be formed easily.

Moreover, since the polyvinylidene fluoride is insoluble in methanolaqueous solution, the electrochemical device may be used as the directmethanol fuel cell which works without the possibility that the catalystlayer peels off during operation and the MEA (Membrane & ElectrodeAssembly) film (composed of the ion exchange membrane and the first andsecond electrodes) breaks during operation.

Therefore, the resulting electrochemical device exhibits improved outputperformance and improved durability in thermal, chemical, andmechanical.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present invention andwithout diminishing its intended advantages. It is therefore intendedthat such changes and modifications be covered by the appended claims.

1-9. (canceled)
 10. A method for producing an electrochemical devicecomposed of a first electrode, a second electrode, and an ion exchangemembrane held between the first and second electrodes, comprisingforming a catalyst layer containing a catalytic substance andpolyvinylidene fluoride; and attaching one or more ion exchange groupsto the polyvinylidene fluoride in the catalyst layer such that thecatalyst layer contains the ion exchange groups that can be used for atleast one of the first and second electrodes.
 11. The method forproducing an electrochemical device as defined in claim 1, furthercomprising bonding the catalyst layer to a precursor of the ion exchangemembrane composed of polyvinylidene fluoride to form a bonded body; andbringing the bonded body into contact with a compound containing the ionexchange groups, thereby introducing the ion exchange groups into thepolyvinylidene fluoride in the bonded body through substitution.
 12. Themethod for producing an electrochemical device as defined in claim 11,further comprising dipping with heating under pressure the bonded bodyin a solution of a compound containing the ion exchange groups, therebyintroducing the ion exchange groups into the polyvinylidene fluorideconstituting the catalyst layer and the precursor of ion exchangemembrane through substitution.
 13. The method for producing anelectrochemical device as defined in claim 12, further comprisinglaminating the first electrode, the catalyst layer, the precursor of ionexchange membrane, the catalyst layer, and the second electrode to forma laminated, and subsequently dipping the laminate in the solution. 14.The method for producing an electrochemical device as defined in claim10, wherein the ion exchange group includes at least one speciesselected from the group consisting of a sulfonate group, a carboxylgroup, a phosphate group, a linear sulfone group, and a perfluorocarbonliner sulfone group.
 15. The method for producing an electrochemicaldevice as defined in claim 10, wherein the catalyst substance containsat least one species selected from the group consisting of platinum,ruthenium, palladium, silicon, carbon, aluminum, magnesium, cobalt,iron, nickel, molybdenum, and tungsten.
 16. The method for producing anelectrochemical device as defined in claim 10, wherein the ion exchangemembrane includes at least one species of ion exchanging materialselected from the group consisting of perfluorocarbon sulfonic acid,non-fluorocarbon sulfonic acid, partially fluorinated carbon sulfonicacid, perfluorocarboxylic acid, non-fluorocarbon carboxylic acid,partially fluorinated carbon carboxylic acid, perfluorophosphoric acid,non-fluorocarbon phosphoric acid, and partially fluorinated carbonphosphoric acid.
 17. The method for producing an electrochemical deviceas defined in claim 10, wherein the ion exchange membrane is prepared tofunction as an electrolyte.
 18. The method for producing anelectrochemical device as defined in claim 10, wherein theelectrochemical device includes a fuel cell.